Asphalt Cement Concrete (“ACC,” or often just “asphalt”) is widely used as a paving material to surface roads, runways and parking lots. By some estimates, up to 90% of all such surfaces are made with ACC. A basic asphalt concrete comprises asphalt (also known as bitumen), a highly-viscous or semi-solid form of petroleum; and aggregates such as stone, sand or gravel, in about a 1:19 ratio (5% asphalt, 95% aggregate). The ingredients are heated, mixed, spread on the surface to be paved (often an earthen, stone or crushed-rock bed) and compacted to form ACC.
A variety of trace ingredients can be added to asphalt cement concrete to improve its strength, durability, performance or construction characteristics. In addition, careful control of aggregate size, shape and composition can significantly improve ACC characteristics. Because of the enormous amount of ACC used around the world, even modest improvements in performance or handling can yield significant benefits.
In the context of a related paving material, Portland Cement Concrete, it is known that the introduction of various types of fibers to the basic Portland cement and aggregate mixture can improve strength and toughness of the resulting concrete. Similar fibers have been used with asphalt concrete to good effect, but differences between Portland cement concrete and asphalt cement concrete's manufacturing and handling requirements make it more difficult to introduce fibers into asphalt concrete. (For example, the elevated temperatures and vigorous mixing required by ACC damages, melts or destroys many fibers that work well with Portland cement concrete, and it is challenging to prevent small, lightweight fibers from blowing away or clumping before they are captured and secured into the asphalt/aggregate mixture.)
One workable method of introducing reinforcing fibers into asphalt concrete is described in U.S. Pat. No. 8,114,514 to MacDonald and Lang. The method is to construct a core containing the fibers, surrounded by an outer container of a polyolefin. This container is introduced into the asphalt concrete during mixing. The elevated temperatures during mixing cause the outer container to melt or disperse, releasing the reinforcing fibers into the AC where further mixing distributes them.
Alternative methods of introducing reinforcing fibers into asphalt concrete may provide greater control over the quantity and distribution of the fibers in the finished pavement, leading to improved pavement characteristics and reduced construction cost.